This invention relates to extruders of the type in which a screw rotatable within a barrel is employed to extrude material from a die connected to the outlet end of the barrel. The invention is concerned particularly with improvements in high output plasticating extruders.
A plasticating extruder receives polymer pellets or powder, works and raises the temperature of the polymer sufficiently to dispose it in a melted or plastic state, and delivers the melted polymer under pressure through a restricted outlet or die. Ordinarily it is desirable that the extrudate be fully melted, uniform in temperature, and substantially free of small gels and other fine structure agglomerations. It also is desirable that the rate of delivery of the molten polymer through the die be regulatable simply by changing the rate of extruder screw rotation and that the rate of delivery at the selected screw speed be substantially uniform.
The basic extruder apparatus includes an elongated barrel which may be heated or cooled at various locations along its length and a screw which extends longitudinally through the barrel. The screw has a helical land on its surface which cooperates with the cylindrical internal surface of the barrel to define an elongaged helical channel. Although the pitch of the screw may vary along the length thereof, it is common at the present time to utilize screws of constant pitch where the pitch is "square", that is, where the distance between adjacent flights is equal to the diameter. The screw is rotated about its own axis to work the plastic material and feed it toward the outlet end of the barrel.
An extruder screw ordinarily has a plurality of sections which are of configurations specially suited to the attainment of particular functions. Examples are "feed" sections amd "metering" sections, which are of basic importance and are present in nearly all extruders for handling thermoplastic polymers.
A typical extruder screw feed section extends beneath and forwardly from a feed opening where polymer in pellet or powder form is introduced into the extruder to be carried forward along the inside of the barrel by the feed section of the screw. In this section the channel depth of the screw is usually large enough to over-feed the solid polymer. This is a desirable effect because the over-feeding action serves to compact and pressurize the polymer particles and form a solid bed of advancing material.
The working of the material generates heat, and melting of the polymer proceeds as the material is moved along the feed section of the screw. Actually, most of the melting occurs near the barrel surface at the interface between a thin melt film and the solid bed of polymer. This general pattern persists until a substantial portion of the polymer reaches the melted state. After some 40 to 70 percent of the polymer has been melted, solid bed breakup usually occurs, and at this time particles of solid polymer become dispersed in the polymer melt. From this point on, it often is advantageous to intimately mix the polymer melt with the unmelted material to accelerate melting and minimize local non-uniformities.
An extruder screw "metering" section has as its special function the exertion of a pumping action on the molten polymer. Ordinarily the throughput achieved by a screw is thought of as being a function of the combination of the "drag flow" and "pressure flow" effects of the metering section.
Drag flow is basically the flow which results from the relative movement between the screw and the internal surface of the extruder barrel. It may be thought of as being proportional to the product of the average relative velocity and the channel cross-sectional area. This drag flow component is directed toward the outlet end of the screw. It may be increased by increasing the speed of the screw and/or by increasing the depth of the flow channel in the screw.
Acting in opposition to drag flow is a pressure flow component stemming from the reluctance of the material to flow through the restricted outlet opening at the end of the extruder passage. The speed of the screw does not directly affect the pressure flow component but of course it may affect such factors as back pressure and material viscosity, which factors, in turn, affect significantly the pressure flow component. On the other hand pressure flow is directly affected by both the depth and length of the screw channel; an increase in channel depth has a tendency to increase greatly the pressure flow component and an increase in channel has a tendency to reduce this back flow component.
In addition to the basic feed and metering sections an extruder screw also may include a number of other distinct sections. Nearly all screws include so-called "transition" sections and some include "vent" sections. Many modern screws also include or have attached thereto special "mixer" sections or devices which provide intensive mixing actions within the at least partially molten polymer. In a typical mixer section, the material is required to move past a barrier of some sort in order to proceed downstream to the extruder outlet. The barrier may, for example, be in the form of one or more lands each having a greater clearance than normal with respect to the extruder barrel surface, and the screw may be so configured that all of the plastic material must pass over the barrier lands. Such barrier lands may be helical or they may extend axially of the screw. Various arrangements of pins and holes also have been employed with some success to achieve intensive mixing.
Over the years there has been a trend toward the use of extruders capable of high outputs. In many applications, various economies in production are possible where high extruder outputs can be obtained on a reliable basis.
However, difficulties have been experienced in increasing extruder outputs. In order to increase the output of an extruder, it is necessary to increase the speed of the screw and/or to increase its channel depth, but both of these approaches may have adverse effects.
An increase in the relative speed between the periphery of the screw and the extruder barrel increases shear rate and consequently has a tendency to raise the temperature of the polymer. The temperature rise may reach runaway proportions, contributing to thermal degradation of the polymer. Further, increases in shear rate and temperature usually have marked effects upon viscosity. A change in viscosity of the material being extruded in turn affects the flow rate of the material through the restricted outlet of the extruder, so that there is a failure to achieve the desired uniformity in output rate.
An increase in channel depth has an opposite effect upon temperature. This is so because the material near the root of a deep channel is not subjected to the same high shear forces as are exerted upon the material adjacent the barrel of the extruder. This low shear level near the bottom of a deep channel is desirable from the standpoint of minimizing the likelihood of runaway temperature increases but it is undesirable from the standpoint of achieving high quality extrudate. After solid bed breakup, some polymer particles remain in solid form for a substantial length of time. The melting of these isolated particles is facilitated by localized mixing actions within the material being fed along the extruder. Such localized mixing depends upon the development of substantial shear forces and these are difficult to develop in the lower regions of deep channels.
Such difficulties are particularly pronounced at high output rates because, in an extruder of a given length, an increase in output rate will be accompanied by a shift toward the outlet of the zone in which the polymer normally exists in solid form. As a result, it is not unreasonable to expect solid particles to find their way along the entire length of a high output, deep channel extruder screw on occasions. The presence of even a small quantity of polymer at the breaker plate or other restricted output zone in an extruder can have important adverse effects upon output uniformity.